The recent shortage of fossil fuels and the rising prices of those fuels has made it increasingly important to conserve energy. Further, the imposition of the 55-mile an hour speed limit has added the force of law behind the efforts to conserve fuel. Thus it is desirable to have a device which will limit the speed of the vehicle in an accurate fashion with a device that is reliable and relatively impervious to tampering by the operator or the mechanic.
Various prior art devices have been proposed but none has been satisfactory for use with diesel-powered vehicles. Prior speed control devices can be grouped into several categories. First are those which act to limit engine speed but which are not useful in limiting vehicle speed since the maximum engine speed in top gear is generally above any vehicle speed limit that would be imposed. The other main group of vehicle speed controllers are those which act to control the linkage of the fuel supply device (fuel injection or carburetor) in an attempt to maintain a predetermined speed. While it has been proposed in U.S. Pat. No. 2,239,328 to partially or completely cutoff the fuel supply when a predetermined road speed has been reached, this previous attempt is not suitable for use with vehicles having injection systems as it does not take into account the special conditions under which such systems must operate. It has been discovered that merely cutting off the fuel supply to the injection pump and injectors will cause those pumps and injectors to become scored and/or seized since the fuel also acts as a lubricant in diesel engines. These parts are highly and carefully machined and it is thus desired to have fuel passing through them at all times.